Day 7: Counting Elements.py

# Counting Elements
#
# Given an integer array arr, count element x such that x + 1 is also in arr.
# 
# If there're duplicates in arr, count them seperately.
# 
# Example 1:
# Input: arr = [1,2,3]
# Output: 2
# Explanation: 1 and 2 are counted cause 2 and 3 are in arr.
# 
# Example 2:
#
# Input: arr = [1,1,3,3,5,5,7,7]
# Output: 0
# Explanation: No numbers are counted, cause there's no 2, 4, 6, or 8 in arr.
#
# Example 3:
#
# Input: arr = [1,3,2,3,5,0]
# Output: 3
# Explanation: 0, 1 and 2 are counted cause 1, 2 and 3 are in arr.


# Constraints:
# 1 <= arr.length <= 1000
# 0 <= arr[i] <= 1000


class Solution(object):
    def countElements(self, arr):
        """
        :type arr: List[int]
        :rtype: int
        """
        t = set()
        c = 0
        for n in arr:
            t.add(n - 1)
        for n in arr:
            if n in t:
                c += 1
        return c